An ichneumon wasp, probably Ichneumon sarcitorius (Hymenoptera: Ichneumonidae) on hogweed. The pattern of black, orange and white bands on the abdomen may seem rather striking but, out on the web, you will also find photos labelled I. sarcitorius that show a distinctly different banding on the abdomen (see here, here and here, for instance). I assume that this is a rather variable species.

Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-09-12.